The present invention relates to a spark plug for an internal combustion engine and more particularly a spark plug of the type in which a spark gap is defined by at least one pair of opposed center and ground electrodes and a fine noble metal tip is welded to the leading end face of at least one of the center and ground electrode main bodies so that energy saving can be attained and long service life can be ensured.
In the spark plugs of the type described above, thin noble metal plates which are made of a platinum alloy such as Pi-Ir,Pt-Rh,Pt-Ni,Pt-Pd or the like and which exhibit high resistance to heat and wear are electrically welded to the spark discharge portions of the center and ground electrodes. A spark plug as described above is diclosed in detail in U.S. Pat. No. 2,296,033.
However, in the spark plugs of the type described, the volume of the center and ground electrodes which define a spark gap is great so that the thermal energy of the flame produced in the spark gap tends to be absorbed by the center and ground electrodes including the thin noble metal plates. As a result, there arises the problem that ignition is adversely affected. Meanwhile, in the recently developed ignition circuits, in order to make an ignition circuit light in weight and less costly to fabricate, both the positive and negative voltages are applied to the spark plug while and negative voltage was applied to the spark plug in the past. The prior art spark plugs have a common defect that the discharge performance is dependent upon the polarity of the voltage applied to the spark plug.
Therefore in order to minimize the flame extinguishing action or effect, when a fine noble metal wire less than 1.0 mm in diameter is electrically welded to the leading end face of an electrode main body, the welded joint between the noble metal tip and the electrode main body is too small in area to provide the required joint strength. Furthermore, the noble metal tip and the electrode main body have different coefficients of thermal expansion so that when the spark plugs are mounted on an engine, the noble metal tips very frequently tend to be separated from the electrode main bodies and drop off in the worse case. In order to overcome the above problems, there has been devised and demonstrated a method in which a noble metal tip is previously formed with an enlarge flange or is fitted into a recess of the electrode main body and thereafter the leading end portion including said recess is caulked and simultaneously the noble metal tip is welded. Such method is disclosed in detail in Japanese Patent Publication No. 56-45264. In either case, the noble metal is used in large quantities and even when the noble metal tips are formed with a flange and electrically welded to the electrode main bodies, separation of the noble metal tips from the electrode main bodies cannot be avoided.